PCI video cards on the 4-series and 5-series (like the DH55HC) boards are not POR(Plan of Record), meaning it was not intended to work as part of the design(they are not validated and/or supported)
Furthermore, when a as per the design of our graphics controllers, when a PCI Express* video card is installed on the computer, the system will electronically disable the integrated graphics controller, therefore it is not possible to use a PCI Express* video controller and the integrated video controller on the processor.
Keep in mind that this motherboard does not have an integrated video controller, it only has video ports, those ports will become active when you use a processor that features Intel® HD Graphics (integrated video chip), otherwise those ports will not work. The following site provides the list of compatible processors for this motherboard and the list of the ones that will require an add-in video card when being used with this motherboard:
So, basically in order to use three monitors on this system you would need to use a video card that provides three pipes of video output.